ASTM F2516

ASTM F2516 specifies a method for measuring the tensile properties of nickel-titanium (NiTi) shape memory alloys, commonly referred to as Nitinol. These materials are widely used in medical devices, such as stents, guidewires, and orthodontic wires, due to their unique superelastic and shape memory behavior.

This test method focuses on characterizing the stress-strain behavior under tensile loading, especially identifying the plateau stress regions that are distinctive in superelastic Nitinol.

Purpose and Scope

The main goal of ASTM F2516 is to determine:

• Upper plateau stress (UPS)

• Lower plateau stress (LPS)

• Residual elongation

• Ultimate tensile strength (UTS)

• Elongation at failure

• Modulus of elasticity (optional)

The standard is primarily used for wire, strip, and tubing made from NiTi alloys and is a critical quality control tool for manufacturers of medical implants and precision components.

Superelastic Behavior of Nitinol

Unlike traditional metals, superelastic Nitinol undergoes large reversible strains (up to 8%) due to a stress-induced phase transformation between austenite and martensite. The hallmark of its stress-strain curve includes:

• Loading plateau: Austenite transforming to martensite

• Unloading plateau: Reversible transformation

• Hysteresis loop: Distinct separation between loading and unloading curves

ASTM F2516 quantifies these behaviors under ambient temperature conditions (22 ± 3°C), where Nitinol is fully in the austenitic phase.

Equipment Required

• Universal Testing Machine (UTM):
  Capable of precise control at low strain rates (0.001–0.01 s⁻¹) with a load cell appropriate for thin wires or small cross-sections.

• Grips:
  Must prevent slippage or damage to the specimen. Pneumatic or wedge grips with compliant clamping surfaces are common.

• Extensometer or Displacement Measurement:
  May use the crosshead displacement if no extensometer is available, but a non-contact extensometer (e.g., laser or video) is preferred for high accuracy.

• Environmental Chamber (Optional):
  If testing needs to be conducted at body temperature (37°C), a temperature-controlled chamber may be used, though ASTM F2516 is typically performed at room temperature.

Specimen Preparation

• Form:
  Wire, strip, or tubing with a gauge length of at least 10 times the diameter or minimum 50 mm.

• Conditioning:
  No special conditioning required unless otherwise specified.

• Number of Specimens:
  At least three specimens per heat lot are recommended for repeatability and validation.

Test Procedure

1. Measure Initial Dimensions:
   Determine cross-sectional area of the wire, strip, or tube.

2. Mount Specimen in Grips:
   Ensure axial alignment and proper clamping to prevent premature failure or slippage.

3. Apply Load at Constant Strain Rate:
   Apply tension at a nominal strain rate of 0.01 s⁻¹ unless otherwise specified.

4. Record Stress-Strain Data Continuously:
   Plot the entire loading and unloading cycle, capturing:

   • Loading plateau (upper stress plateau)

   • Unloading plateau (lower stress plateau)

   • Permanent set or residual strain

5. Optional – Perform Cyclic Loading:
   Some users perform multiple cycles to evaluate fatigue or stabilization behavior.

Key Calculations

Stress (σ):

σ = F / A

Where:

• σ = Engineering stress (MPa or psi)

• F = Applied force (N or lbf)

• A = Cross-sectional area (mm² or in²)

Strain (ε):

ε = ΔL / L₀

Where:

• ε = Engineering strain (dimensionless or %)

• ΔL = Change in length

• L₀ = Original gauge length

Residual Elongation:

Measured after unloading to zero force:

Residual Elongation (%) = [(L_final - L₀) / L₀] × 100

Reporting Requirements

The ASTM F2516 report should include:

• Sample description (wire, strip, tubing; dimensions; alloy type)

• Test temperature

• Strain rate used

• Stress-strain curve with indication of:

  • Upper plateau stress (UPS)

  • Lower plateau stress (LPS)

  • Ultimate tensile strength (UTS)

  • Residual elongation

  • Strain at break

• Number of cycles (if cyclic testing performed)

• Failure mode and any visual observations

• Heat lot identification

Applications

ASTM F2516 is widely used in the medical device industry, especially for:

• Orthodontic archwires

• Endovascular stents

• Heart valve frames

• Guidewires and catheters

• Shape memory actuators and couplings (in aerospace and robotics)

Similar and Related Standards

Here are similar or complementary test methods used with shape memory or superelastic alloys:

• ASTM F2004 – Test Method for Transformation Temperature of Nickel-Titanium Alloys by Thermal Analysis
  (Used to determine the temperature range at which phase transformation occurs)

• ASTM F2082 – Test Method for Determination of Transformation Temperature of Nitinol Alloys by Bend and Free Recovery
  (Alternate method to F2004, often used for quality control)

• ISO 15841 – Dentistry – Wires for Use in Orthodontics
  (Covers mechanical and superelastic properties of orthodontic wires)

• ISO 10993 – Biological Evaluation of Medical Devices
  (Often paired with F2516 for biocompatibility of NiTi implants)

• ASTM E8 / E8M – Tension Testing of Metallic Materials
  (General metal testing, but not suited for superelastic behavior)

Summary

ASTM F2516 provides a precise and standardized way to measure the tensile behavior of superelastic Nitinol alloys, particularly focusing on the unique plateau stresses and recoverable strain behavior that define these materials. It is critical for validating the performance and safety of medical implants and devices, where consistent superelastic behavior is required.

By following this test method, manufacturers and engineers can ensure that NiTi components meet design specifications and perform reliably under stress in clinical or mechanical applications.